In recent years, communications networks have proliferated and become increasingly complex and difficult to manage. Whilst network management centres are able to implement most functions of network management in a centralised manner, centralised management is not practical whenever fast responses are required, such as responding to failures and setting up and down connections which may need to be done on relatively rapid timescales.
For functions which require actions to take place on such relatively rapid timescales, decentralised management can be used, which enables faster response times to failures, and enables large and complex networks to be more easily managed. Different domains of a networks may thus be managed by different, often autonomous, network controls, for example network operators, who have no visibility of the behaviour and performance characteristics of the network outside their own domain.
This partitioning of the network management can create problems. For example, it may no longer be possible for a network operator controlling the network elements and links at the optical network layer to assess how any optical degradation is affecting the quality of service provided in higher layers, for example in the client layer network's electronic domain.
Optical degradation can occur rapidly or slowly, and is particularly likely to occur when the photonic or all optical network layer of a communications network uses Dense Wavelength Division Multiplexing (DWDM), for example, when a new wavelength channel is injected into an optical link carrying an existing wavelength channel. Channels in a DWDM system are closer in wavelength than in systems using a simpler Wavelength Division Multiplexed transmission technique, and so are more likely to experience cross-talk related degradation due to the narrower margins provide between channels. (See FIG. 1 of the accompanying drawings for example, which shows how adjacent channels of wavelengths λ1, λ2  experience cross talk when their spectra overlap.)
The extent to which any degradation in the optical layer of a communications network is affecting the quality of the signals transmitted in a partitioned, client layer network (see for example, FIG. 2) is not easy to determine within the optical layer without reference to the client layer. Consequently, the extent to which optical degradation is affecting the signals transmission quality in the optical domain may not be initially apparent to a network operator who has visibility only of the optical network. In such cases, only when the degradation in the optical layer has fallen to such an extent that alarms are triggered in the client layer network will the optical layer network operator be informed that the problem exists and that action must be taken to restore the quality of service in the client layer network. This would constitute a non-compliance of the service level agreement between the network service provider and the network client.
In a transparent optical network, it is not possible to access overhead bits in transmitted data to obtain performance related data. Thus it is difficult to monitor bit error rate (BER) within the optical layer. Conventionally, transmission-related parameters that can be monitored include optical power levels, optical signal-to-noise ratios, temperature, electrical BER and various other parameters associated with the electronics present at transmitters optical amplifiers and receivers. Such parameters are then used by the network management system to provide guaranteed BER performance to the client layer network. However, such parameters cannot be determined at nodes where it is not possible to access the client layer network.
Conventional BER detection schemes typically assess BER in the electronic domain, and initiate action in the optical domain only when the electronic BER has exceeded a predetermined critical level. Usually, therefore if a network operator has no visibility of, and/or network management cannot monitor, the BER in the electronic domain, the quality of service in the electronic domain may be compromised before actions can be taken in the optical domain to remedy the situation.
US patent application co-filed herewith entitled “IMPROVED METHOD AND APPARATUS FOR MEASURING OPTICAL TRANSMISSION CHARACTERISTICS IN PHOTONIC NETWORKS”, Nortel ref: 14780ID (hereinafter referred to as “copending application”), and incorporated into this application by reference, describes a technique which enables degradation in an optical link which results in distortion to be distinguished from degradation in the link which gives rise to noise. This technique thus enables a proxy for the electronic behavior in the client layer network to be provided within the optical layer network.